Spectrum+ 48k repair

[Slauri]

Okay

[1024MAK]

A 48K ZX Spectrum draws between 600mA and 700mA (with no expansions connected). 700mA is half the rated capacity of the Sinclair ZX Spectrum 9V 1,4A PSU.

The output of the PSU is required not to drop below 9V at the full load current of 1.4A. The output voltage is not specified at other output current levels.

When connected to a ZX Spectrum, the PSU output voltage is usually between 10V and 12V. You can measure this on the left hand leg / pin of the 7805 voltage regulator chip. Use the metal heatsink tab as the 0V/GND/ground point for your meter.

If the PSU you have been using has similar characteristics, then it is unlikely to have damaged your Spectrum. However, issue 2 boards really don’t like an input voltage of 11.9V or greater (unless they have had their on board DC/DC converter circuitry upgraded).

Mark

[Slauri]

Nevermind, my universal power supply has a 9 volt option too.

[Slauri]

The power supply might actually be the culprit, because the picture is much different after swapping out the entire lower ram and the cpu. It still shows garbage but is it normal for the ULA to heat up in the center in under 30 seconds?

[Slauri]

Oh, it heats up in under 15 seconds.

[Slauri]

Is there any way to test the ula?

[Ast A. Moore]

Is there any way to test the ula?

Sure, in a known-good machine. Keep in mind that ULAs are usually only backward compatible, so a later-issue ULA will work in all the previous Spectrum models, but that might not be the case the other way around.

[Slauri]

Any other ways?

[1024MAK]

If the machine is generating a video output /even if it’s full of different coloured squares and other ‘garbage’), that’s most of the ULA working (the main function of the ULA is to generate the video display, produce the CPU clock, deal with the keyboard and interface to the speaker ear and mic sockets.

If the area of the ‘lower’ RAM (4116 or equivalent) contains garbage data, the ULA does not care, it will read and then display this ‘information’. It’s the code running from the ROM that clears the memory and sets up black text on a white background.

Mark

[Slauri]

I tested the voltage on the ROM and it was good. I'll replace the tr4 too. Is there any equivalents to the ZTX 650 transistor?

[1024MAK]

I tested the voltage on the ROM and it was good. I'll replace the tr4 too. Is there any equivalents to the ZTX 650 transistor?

ZTX651, or ZTX653

Mark

[Slauri]

Can i replace the ZTX213 transistor with ZTX750 or ZTX753?

[1024MAK]

Before you change anything else, can you please post up what voltages you get on the +9V, +5V, +12V and -5V rails. I posted details on how to do this here.

And yes, TR5 (ZTX213) can be replaced with a ZTX750 or ZTX753 (as well as many other small signal PNP types although the lead out may not be the same as the ZTX213 that was originally fitted).

Mark

[Slauri]

On the ULA pin 13 there is only 3.17 volts. On the pin 14 there is almost perfect 5 volts. Is this normal?

[Slauri]

The picture changes when the ULA heats up.

[1024MAK]

The voltage on pin 13 of the ULA will be lower than the voltage on the +5V rail due to resistor R34 (15 ohms).

Internally the ULA operates at a much lower supply voltage (less than 1.8V).

I can’t remember what the normal voltage is on pin 13, I would have to test one of mine. But I am not in a position to do that today.

The picture changes when the ULA heats up.

In what way?

Mark

[Slauri]

The machine had been on for a while already. I pressed the reset button multiple times and this happened: https://youtu.be/PVr2weWOylE

[1024MAK]

The ULA is displaying the random content (garbage data) of the lower (4116) DRAM. But although the CPU looks to be running, it is not able to write new data to the lower (4116) DRAM. Hence the program in the ROM is unable to clear the memory and generate the normal screen display.

You can tell that the CPU is running, by the border mostly being white (one of the first things the program code in the ROM does after power up / reset is to turn the border white (although it sometimes get described as light grey) and later on, for some random reason (likely due to the random data in RAM) it shows moving “loading” bars (stripes) in the border area.

You also do not appear to have any colours on screen.

I’ve ran out of time for now. If I remember, I post some more tomorrow.

Mark

[Slauri]

>>You also do not appear to have any colours on screen.
I was just adjusting the trims before I recorded the video.

>>You can tell that the CPU is running, by the border mostly being white.
Yes. Before i replaced the CPU or the lower ram the border would stay black and it wouldn't do anything when I pressed the reset button.
Before: https://youtu.be/mGKvQZV_2P4
After: https://youtu.be/PVr2weWOylE

[Slauri]

I noticed an another problem. If the machine had been on for a while the top section of the screen would start to flicker a bit, and after a few minutes the top section would go nuts. It just went out of sync and flickered even more. At that point I disconnected the power cable and cooled down the voltage regulator and the ULA with some compressed air. I turned the machine on again. Now the bottom section was going crazy too. I let the machine cool down for half an hour and the flickering was gone.

[llopis]

I haven't read the whole thread, but did you change the capacitors? With problems like these, you might as well start there. That way you can rule that out as a potential problem.

[Slauri]

Recapping was the first thing i did.

[Slauri]

Hi everyone, i got 2,98 volts on the pin 1 of the lower ram. The other voltages were good.

[Firefox]

That should be -5V, so that's obviously not good! No wonder your lower RAM isn't working.

I was looking at how the voltage generation circuity functions just the other day, but it's already evaporated out of my head...

The alternative to fixing the -5V generator is to fit a lower RAM replacement board, which doesn't need -5V.

[Slauri]

I already have the lower ram replacement board soldered in